The present invention relates to a display device with a memory making it possible to enter or print out in a sensitive material symbols or pictures which are then observed either directly or by projecting on to a screen. These symbols or pictures remain entered in this material until an external action erases them. The invention also relates to display systems incorporating such a device.
It is known to construct display devices which use liquid crystals as the sensitive material. Such devices are more particularly described in the work entitled "Opto-electronique" by Georges Broussaud, and whose ISBN reference is 2-225 39 138-6.
In such a device it is known to use a smectic material previously brought into a phase where it is diffusive and to make it transparent by applying thereto a direct or alternating electrical field. Such a device is more particularly described in French patent application 74 20715 filed by the Applicant company on June 14th, 1974 and entitled "Black and White Image Reproduction Device using a Material having a Smectic Phase and Remote Transmission and Remote Reprography System utilizing this Device."
In this device the liquid crystal is made diffusive by a light source, whose beam is focused on the device so as to heat the liquid crystal. In the absence of an electric field when the latter cools it becomes diffusive, whereas when such a field is applied during cooling it becomes transparent to a greater or lesser extend depending on the field value applied. To obtain this light beam in practical constructions a laser, together with a system of electro-optical deflectors, are used. These members are cumbersome and costly and do not make it possible to obtain a display device which can be mass-produced.